We showed direct evidence using ESR analysis of the sequestration of heme-nitrosyl complexes within cardiac allografts prior to rejection using an improved tissue processing technique. We showed that heme-nitrosyl signals consistent with nitrosylmyoglobin is a prominent feature which is specific for tissue prior to rejection of cardiac allografts and not seen in isografts. This nitrosylmyoglobin signal in allografts is decreased by prior treatment of recipients with the antioxidant PDTC. As myoglobin is important to releasing oxygen for normal cardiac respiration and function, further studies will be necessary to evaluate whether decreases in nitrosylation of myoglobin contribute directly to amelioration of myocardial contractile dysfunction in cardiac allografts and to improvement in graft survival.